In the field of electrical circuit application, a reference voltage is usually provided by using a reference voltage generator such as a bandgap voltage generator. However, in a conventional reference voltage generator, a control voltage may be as low as 1.2 volt. Then, an operational amplifier may be used to amplify the 1.2 volts voltage to generate a 3.3 volts reference voltage. In this sort of circuit structure, an operational amplifier is necessary, and a compensation capacitor of the operational amplifier will make it difficult to reduce circuit area. In addition, it is also difficult to improve the power supply rejection ratio (PSRR) when using a conventional reference voltage generator. In other words, the provided reference voltage is easily affected by noise of a power supply. When manufacturing a conventional reference voltage generator, limitation of process is a problem. For example, it is hard to use a Gallium arsenide (GaAs) Heterojunction Bipolar Transistor (HBT) process. For example, in a conventional reference voltage generator, a current source including P-type metal oxide semiconductor field-effect transistor (MOSFET) is often required, and a circuit including N-type MOSFET is also needed. Hence, a GaAs process is hard to be used in manufacture.